Transducing system



y 3, 1952 'w.- L. BOND TRANSDUCING SYSTEM Filed July 2, 1946 FIG. I

RELAXATDN FRA'OUCNCY FREQUENCY \rtsat wvawmp W L. BOND ATTORNEY PatentedJuly 8, i952 UNITED STATES PATENT OFFICE Telephone Laboratories,Incorporated,

New

York, N. Y., a corporation of New York Application July 2, 1946, SerialNo. 680,965

9 Claims. I

This invention relates in general to vibrating systems. and inparticular to electro-acoustic vibrating systems which include contactmedia having frequency selective properties.

The primary object of this invention is to facilitate the transfer ofelastic vibrations from one element to another in an acoustic system.

In accordance with the invention, acoustic contact is maintained betweenthe cooperating elements of a vibrating system by means of a mass ofsubstance interposed between such elements which has the property ofconforming to the shapes of the respective contact surfaces in responseto slow variations in the applied pressure and responding elastically torapid variations in pressure. 'The slow variations in pressure are ofsuch a nature as would occur in the ordinary process of maintaining twosurfaces in contact; while the rapid variations in pressure are of thenature of applied sonic or supersonic vibrations. A preferred embodimentof such contact medium comprises a material known as bouncing puttywhich is described as a dimethyl silicone reaction product, and which isdisclosed and claimed in application Serial No. 569,647, now Patent2,541,851, entitled Composition of Matter" filed on December 23, 1944,in the name of James G. E. Wright.

The above described contact medium has certain important advantages overthe contact media of the prior art, in that it is frequencyselective,transmitting only frequencies above a predetermined critical frequencyrange and suppressing frequencies below such range. It is easilyapplied, easily removed, and maintains eiiicient contact between any twosurfaces, including those of odd or non-conforming shapes. Furthermore,it does not appreciably penetrate and thereby change the vibrationalcharacter of the contact surfaces. Moreover, vibrations transmitted inaccordance with this invention sustain much lower energy losses than isusual in prior art systems utilizing oily or waxy contact agent in theconventional manner.

For the purpose of illustration, the present invention will be describedas embodied in an acoustic system for detecting flaws in propellantexplosive grains comprising cylinders of the order of 8 inches inlength. Such a testing system comprises the following cooperating parts:a test propellant grain disposed on a support, a piezoelectricvibrations generator centrally located with respect to the ends of thecylindrical grain and acoustically coupled thereto by means of a mass of"bouncing putty or similar material interposed between the contactingsurfaces, two piezoelectric vibration responsive devices symmetricallydisposed with respect to the vibrations generator and contacting thegrain by means of attached steel rollers. and a cathode-ray oscilloscopehaving horizontal and vertical deflection plates respectively connectedto the piezoelectric responsive devices.

My invention will be better understood after a study of the detailedspecification as set forth hereinafter and the attached drawings, ofwhich:

Fig. 1 shows rigidity versus frequency characteristic for a typicalacoustic contact material in accordance with the present invention;

Fig. 2 shows an enlarged view of an acoustic contact agent of thepresent invention interposed between the contact surface of apiezoelectric unit for generating or detecting elastic vibrations andthe surface of a vibrating body; and

Fig. 3 shows a piezoelectric system for the acoustic testing ofpropellant explosive grains in which a piezoelectric vibrationsgenerator is coupled to the test surface in a manner prescribed by thepresent invention and shown in the detailed drawing of Fig. 2.

One of the most difficult problems encountered in the eificient designof electro-acoustic systems is that of maintaining a constant andreproducible contact with the vibrating surface through which thevibrational energy can flow without excessive losses. In applyingpiezoelectric units to test surfaces for the purposes of generating ordetecting vibrations, it has been found that the vibrational responsevaries over a wide range with relatively small changes in the pressureof application. Such a contact between respective vibrating surfacesactually takes place at a number of points, each of which consists of asmall area, usually of microscopic dimensions, at which the material ofthe contacting bodies is elastically or plastically deformed. Up to acertain point, which depends on the materials and surface contours, thiscontact area increases rapidly with pressure and then less rapidly asthe pressure is further increased.

The need for high and constant contact pressures in order to eliminatevariations in vibration transmission between the elements of anelectroacoustic system is largely obviated by the use of a contactmedium that has a putty-like consistency to touch but behaves in themanner of a solid to quick deformations. Such a material is easilyplaced in position, and at frequencies in the audio and super-audioranges behaves like a rigid connection of large area.

As hereinbefore stated, a preferred embodiment of such contact medium isa substance known as bouncing putty which is described and claimed inapplication Serial No. 569,647, supra. The substance there disclosed isa putty-like, elastic, plastic composition comprising a dimethylsilicone reaction product, more particularly a heat reaction product ofa dimethyl silicone oil and a boron compound, specifically a pyroboricacid.

Another substance which serves suitably as a contact medium inaccordance with this invention is a sodium silicate NazSiO; described inan article entitled Bounces Like Rubber appearing on page 199 of theScience News Letter of September 29, 1945. This substance is made fromone of the highly silicious silicates from which water has beenevaporated until it composes only about 65 per cent of the solution.

The teachings of the present invention are not limited to the twospecific agents mentioned. but are equally applicable to other materialshaving similar properties.

In order for a material to be so characterized that it respondsplastically to slowly applied stresses, and in the manner of a solidbody to rapidly applied stresses, such as sonic or supersonicvibrations, it should preferably exhibit certain well-defined changes inrigidity with progressive changes in the frequency of impressed elasticvibrations.

For the purposes of this specification and the attached claims, rigiditymay be defined as that property of a body by which it resists change inshape. The rigidity of a body is measured by the ratio of the appliedtangential distorting stress to the distortion it produces.

Referring to Fig. 1, a substance such as bouncing putty," which issuitable for the uses of this invention, has a rigidity vs. frequencycharacteristic which assumes a relatively low, nearly constant valueover the lower range of frequencies. When a certain critical range offrequencies is reached, the characteristic rises rather sharply to arelatively high saturation value which remains nearly constant forfurther increases in frequency.

That frequency at which the rigidity of a particular substance reacheshalf its ultimate value will be defined for the purposes of thisspecification and the attached claims a the relaxation frequency of suchsubstance.

In one aspect, this invention comprises the use of a thin layer ofmaterial having a relatively low relaxation frequency as a vibrationalcontact agent in an acoustic system which is vibrating at a frequency orrange of frequencies substantially above the relaxation frequency ofsuch agent. All liquids have relaxation frequencie in accordance withthe above definition; but in most cases, such frequencies are beyond therange of usefulness for th purposes of this invention. Although oils andsimilar materials have frequently been used in prior art systems asacoustic contact agents, such use has been at frequencies of vibrationsubstantially below their respective relaxation frequencies. Asignificant feature of my discovery is that above their relaxationfrequencies, such contact mediabehave in the manner of solids, whilebelow their relaxation frequencies, they behave in the manner ofliquids.

The application as a contact agent in an electroacoustic system of amaterial of the nature described in the foregoing paragraphs is shown inFigs. 2 and 3 of the drawings.

Fig. 2 shows a piezoelectric unit I, which may Ill) constitute anelement of either a vibration generator or detector, maintained incontact with the surface of a vibrating test member 2 by means of aninterposed mass 3 of bouncing putty or such similar acoustic contactmedium as hereinbefore described. For optimum performance in thepresently described system, a putty mass 3 about the size and shape of apea is initially interposed between the contact surfaces. A pressure offrom ten to twenty pounds per square inch is then applied for a periodof about a minute in order to flatten the contact element 3 into apancake shape having an approximate thickness of M of an inch-and anapproximate diameter of of an inch. Once properly applied, the puttymass 3 acts in the manner of a glue in holding the piezoelectric unit Iin contact with the surface 2, so that only enough pressure is requiredbetween the contacting surfaces to prevent the putty mass 3 frombecoming dislodged during vibration. For this purpose, the weight of thepiezoelectric unit I resting on the surface 2 is usually sufficient.Alternatively, the mass 3 may be shaped before application to the testsurface 2.

Because of the nature of the putty contact element 3, the system isoperable under a wide range of contact pressures. However, it isapparent that too great contact pressure will cause.

the contact layer 3 to spread too thin for satisfactory operation.

The piezoelectric unit I, which is used for the purposes ofillustration, comprises a group of 45-degree Z-cut crystals 4, ofammonium dihydrogen phosphate, each provided with evaporated goldelectrodes on both faces, and all cemented together to form a prism 1inch square by 2.5 inches long. Odd and even electrodes are respectivelyconnected to the terminals 5 and 6 comprising thin pieces of gold-platednickelsilver on opposite ends of the prism, whereby the respectivecrystal plates 4 are connected in parallel to form a condenser ofapproximately 675 micro-microfarads capacitance.

When used in a fixed position as a vibrations generator or detector, thegroup of crystals 4, is first cemented to a thin ceramic late I, whichis in turn cemented to an iron prism 8 which is 1.9 inches long and ofthe same cross-section as the crystal group and which serves as a highimpedance to the vibrations of the crystal unit I. Integral with theprism 8 at the cemented end is a flange 9 which serves as a supportingmember whereby the unit I may be attached to a carriage for maintainingit in contact wtih the surface 2.

The surface of the unit I, which is brought to bear on the test surface2 through the contact medium 3, is surmounted by an iron shoe I0, 1 inchsquare by A-inch thick, cemented thereto. The entire crystal unit isthen surrounded with a thin loose sheet of metal foil for electrostaticshielding. When in-firm contact with a nonresonant solid, the mainlongitudinal resonance of the unit I is about 17 kilocycles.

If merely clamped onto the surface 2 of the piece to be oscillated, asin prior art practice, the piezoelectric unit I, which oscillates byelongation and contraction along its length thousands of times persecond, would push but not pull the test surface 2, thereby dissipatinga large proportion of the vibrational energy. However, by means of theputty contact 3, the piezoelectric unit I is enabled to both push andpull, effectually oscillating as part of the test element, and therebytransmitting a much higher percentage of vibrational energy. Efiicientvibrational cog-r tact is maintained by the mass 3, notwithstanding theshapes of the contacting s1 irfaces of the piezoelectric unit I and thetest'element 2 which may be odd or non-conforming.

A piezoelectric firfit' coupled to a test surface in the mannerdescribed in the preceding paragraphs with reference to Fig. 2 may beincorporated in different types of electro-acoustic com; binations.However, for the purposes of illustration, the piezoelectric unit I andthe interposed contact medium 3 will be described as functioning partsof a vibration generator in a system for locating flaws in largepropellant explosive grains, such as shown in Fig. 3 of the drawings.The typical acoustical test system of Fig. 3 and the component elementsthereof are used merely for the purposes of illustrating the presentinvention; and it will be apparent to those skilled in the art that theteachings of this invention are equally applicable to acoustic systemsof totally different construction and function.

In the process of manufacture of large propellant grains, which comprisea mixture of pulverized crystalline materials compressed hydraulicallywith a small amount of binder, flaws occasionally develop which resultin dangerously high pressures arising during combustion in a motorchamber. The function of the apparatus shown in Fig. 3 is to locate suchflaws by impressing sonic frequencies of from 2 to 40 kilocycles on thetest grain, and observing irregularities in the vibrational pattern ofthe grain by means of a pair of symmetrically located compressional waveresponsive devices which impress their respective outputs on thedeflection plates of a cathode-ray oscilloscope.

A standard test grain II of the composition described, cylindrical inshape with a length of 8 inches and a diameter of 7 inches, has an axialperforation extending from one end to the other thereof, into which isinserted a supporting member I2 comprising a length of 2-inch machinediron pipe 2 which is rigidly supported in a horizontal position by meansof an angle bracket I3 attached to a heavy wooden base It. Ifhe pipe I2is provided with two perforated 3-inch tapered rubber stoppers Ia andI51) which are forced into the grain perforation from either sidethereby providing a rubber vibration insulation so that the supportingstructure does not appreciably interfere with the free vibrations of thegrain II. The perforated stoppers I5a and I5b are shrunk into collars16a and IE1), respectively, which fit over the pipe I2 forming bearingsadapted to rotate and slide thereon. thus enabling the grain II to berotated to different angular positions about the pipe I2.

The grain II is driven to vibrate elastically by means of thepiezoelectric vibration unit I,

which is similar in construction to the piezoelectric vibration unit Idescribed hereinbefore with reference to Fig. 2. Vibrational contact ismaintained between the surface of the test grain II and the unit I bymeans of a mass of material 3' which may comprise bouncing putty or suchother suitable acoustic contact material as hereinbefore described, andwhich is similar in size and shape to the mass 3 described withreference to Fig. 2.

The vibration generating unit I is supported in a central position withrespect to the two ends of the grain II by means of a clamp attachmentbetween the flange 9' and the bracket I1, which is rigidly attached tothe collar I6a riding on the A- P 6 rod so that the watirsnaitdllmanerotated with the grain l l During ordinary conllififihs bf operation,the bracket I1 is preferably positioned so that the piezoelectricgenerating unit I is pressed against the surface of the test grain IIwith a force equal to its own weight. For the purposes of initialapplication and shaping of the contact element 3', somewhat greaterpressures may be desired, as hereinbefore described, in which casesuitable weights may be hung on the bracket I1, and removed once thecontact has been secured.

The generating unit I is energized to vibrate 7 piezoelectrically bymeans of connections througli the electrode terminals 5 and its mate(not shown) to a circuit which includes a conventional oscillationsgenerator I3 and the amplifier I9.

The longitudinal elastic vibrations which are induced inthegrain IIthrough the contacting mass 3 by means of the centrally locatedgenerating unit I and its associated circuit are detected by vibrationresponsive units 20 and 2| which are symmetrically positioned withrespect to the ends of the grain I I and the vibrations generating unitI.

The vibration responsive units 20,, a n d Zly single crowned steel rgller h aving a diameter of V inch. and of an inch wide, and with itsaxis disposed parallel to the axis of the grain II, so that it movesover the surface as the grain rotates.

The units 20 and 2| are held in place by means of clamp attachments tothe respective arms 24 and 25 rotatably disposed on the horizontal shaft26 which is rigidly connected to the support I3. Thus, it is possible tobring the crystal units 20 and 2I in contact with the surface of thegrain I I in a definite way which can be duplicated.

Cracks or non-homogeneities in the grain II produce dis 'rifiIa' siii'tli"vibratiohal Ye fifasi'isiijiniififiia-znfa izfrrfi fi fi" wcompared on a recording instrument such as the cathode-ray oscilloscope21. The output of the left-hand responsive unit 20 is connected throughits electrode terminals to the horizontal deflecting plates of theoscilloscope 21; while the output of the right-hand responsive unit 2|is connected through its respective electrode terminals to the verticaldeflection plates 29 of the oscilloscope 21. If the grain ishomogeneous, and the units 20 and 2| symmetrically positioned, theiroutputs will be equal, and will therefore produce a diagonalstraight-line pattern on the luminous screen of the oscilloscope 21. Ifthe grain is non-homogeneous, the elliptical pattern traced on thescreen of the oscilloscope 21 will have coordinates indicative of thesize and position of the discontinuity.

While the positioning of the recording units 20 and 2| on the curvedsurface of the grain II is not critical, excepting for the stipulationthat they be symmetrically placed with respect to the ends thereof andto the vibration unit I, it has been found that optimum recordings areobtained when the generating device I is rotated through a verticalangle of degrees with respect to the responsive devices 20 and 2|.

In a system in which the responsive devices 20 and 2I are to be heldstationary, rather than brational energy is transmitted ,from the testgrain to the respective responsive devices 20 and 2| with a minimum ofloss.

What is claimed is:

1. A system comprising a plurality of elastically \x \vihrator elementsincludifigavibiation generatcnreo'fiiiied in vibration transmittingrelation to each other, two of said elements being coupled 'to eachother by a contact agent comprising a putty-like mass having theproperty of conforming to the shapes of the contact surfaces of saidelements in response to slowly applied stresses and reacting elasticallyin response to rapid variations in stress induced by the vibrations fromsaid generator.

2. In a system in accordance with claim 1, said contact agent comprisinga dimethyl silicone reaction product specifically a pyroboric acid.

BTAsystemcomprising in combination a vibrations generator, an elasticbody, and interposed between said generator and said body a putty-likemass having the property of conforming to the shapes of the contactsurfaces of said generator and said body in response to slowly appliedstresses and reacting elastically in response to rapid variations instress.

4. A system in accordance with claim 3 in which said mass comprises adimethyl silicone reaction product specifically a pyroboric acid.

5. A system comprising in. combination an elastically vibrating body,means responsive to the vibrations of said body, and a contact agentinterposed between said body and said responsive means, said contactagent comprising a puttylike mass having the property in response toslow variations in stress of conforming to the shapes of the contactsurfaces of said body and said responsive means, and said mass havingthe property of reacting elastically in response to rapid variations instress.

6. In a vibratory system, a contact agent for acoustically coupling theelements of said system interposed between said respective elements,said a ahnuwaw u contact agent having a relaxation frequencysubstantially below the minimum frequency of said vibratory systeru.

'7. A system in accordance with claim 6 in which said contact agentcomprises a dimethyl silicone reaction prodract, spec-ifidally apyroboric acid.

8. in a system vibrating longitudinally within a given frequency range,said system comprising at least two elements, a mass of materialinterposed between and contacting said elements, said interposed masshaving a relatively high rigidity in response to impressed frequenciesabove a certain critical range of frequencies and a relatively lowrigidity in response to impressed frequencies below said critical rangeof frequencies, wherein said critical range is below the range ofvibration of said system.

9. A system comprising a plurality of vibratory elements in vibrationtransmitting relation, wherein one of said elements is connected invibration driving relation to another of said elements through acontact. agent comprising a putty-like mass having the property ofconforming to the shape of the contact surfaces of said elements inresponse toslowly applied stresses below a critical range of frequencyand reacting elastically in response to rapid variations in stress abovesaid critical range of frequency, wherein the vibrations applied by saiddriving element include a frequency above the said critical frequencyrange of said contact agent.

WALTER L. BOND.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,280,226 Firestone Apr. 21, 19422,431,878 McGregor Dec. 2, 1947 2,458,581 Firestone et al. Jan. 11, 1949OTHER REFERENCES Publication: Rubber Age, November 1944,

pages 1'73, 1'74 and 175.

Publication: The Oil and Gas Journal," October 6, 1945, pages 86, 8'7and 88.

